This application is based on Japanese Patent Application Nos. 2000-251126 filed Aug. 22, 2000 and 2001-135152 filed May 2, 2001 the content of which is incorporated hereinto by reference.
1. Field of the Invention
This invention relates to a micro-fluidic cell for optical detection of gases for use in a sensor for detecting trace substances present in the air, a method for producing the micro-fluidic cell for optical detection of gases, and a gas trapping cell for use in the micro-fluidic cell for optical detection of gases. The present invention also relates to an apparatus for optical detection of gases which is equipped with the micro-fluidic cell for optical detection of gases.
2. Description of the Related Art
For analysis of organic gases causing air pollution, the concentrations of the gases to be analyzed (may hereinafter referred to as target gases) are generally very low. Thus, gas trapping treatment of the low concentration target gases is necessary at a stage prior to the analysis operation. Conventional concentration and analysis devices most widely use a device for adsorbing target gases to an adsorbent loaded in an adsorbing collection tube, and then applying thermal desorption treatment to recover the target gases as concentrated gases with high concentrations, and introducing these gases into the analyzer.
The procedure for use, and the problems with, the conventional concentration and analysis devices will be briefly described below, with a gas analyzer comprising a combination of analysis means, such as an ultraviolet (UV) spectrophotometer (Japanese Patent Application Laying-open No. 2000-241313), and a method comprising gas chromatography and cold trapping, being taken as examples.
At a site for which analysis should be made, the air containing organic target gases is introduced into a collection tube, and the gases are collected into the adsorbent. Then, the collection tube is heated to desorb the organic gases adsorbed to the adsorbent as concentrated gases. Then, the concentrated gases are introduced into an analyzer, such as an ultraviolet spectrophotometer, if it is used as means for analysis. If a gas chromatograph is used as means of analysis, the concentrated gases are recovered again into a cold trap device in which liquid nitrogen is circulated, whereafter the concentrated gases are reheated quickly, and introduced into the gas chromatographic analyzer.
In these procedures, the following problems arise: The collection tube so far used has dimensions as large as several millimeters in diameter and several tens of centimeters in length. Thus, when the gases are recovered by desorption upon heating, the adsorbent is heated uniformly, and the recovery by desorption does not give a sharp response. As a result, the concentration effect declines, and sensitivity lowers. The thickness of the tube wall is as large as 1 mm or more, so that when the outer wall is heated with the heater, a temperature difference occurs between the heater and the adsorbent loaded in the tube, decreasing temperature control accuracy. When the heating temperature of the adsorbent is controlled to an appropriate desorption temperature in each target gases to separate them into components, the large volume of the collection tube decreases the accuracy of temperature control, thus leading to poor separation into the components. When the heating portion is large, a high heating temperature and a long heating time need to be set in order to heat the entire adsorbent to a temperature necessary for desorption of the gases. Hence, the problem of an increased electric power consumption is also caused.
Moreover, the internal diameter of the collection tube is as large as about several millimeters. Thus, there is need to put quartz wool, rid of interfering components, ahead of and behind the adsorbent to set the adsorbent in place. When a powdery adsorbent is used, the adsorbent enters between the fibers of the quartz wool, varying the cross section area of contact between the adsorbent and the gases. This may cause a decrease in the measurement accuracy.
Furthermore, when the concentrated gases are introduced into the analyzer such as an ultraviolet spectrophotometer, the concentrated gases diffuse in the detection cell, if the inner volume of the detection cell is as large as about several tens of cubic centimeters relative to the optical path length of 10 cm. As a result, the concentration effect declines, and the sensitivity lowers. The large size of the detection cell also poses the problem that the concentration of the gas becomes different between the wall surface and the center of the cell, causing an error of quantitative measurement.
Besides, when the concentrated gases are recovered into the cold trap device, the high temperature gases are cooled and recovered as described above. Thus, the cold trap device needs to be cooled by circulation of a refrigerant such as liquid nitrogen. For this purpose, a refrigerant reservoir with a capacity of about 10 liters, and a refrigerant circulator are required, making the scale of the apparatus large. The cold trap device solidifies the gases to recover them. The gases need to be reheated for recover, thus requiring an additional form of heating system.
The present invention has been accomplished in consideration of the above circumstances. It is an object of the invention to provide a micro-fluidic cell for optical detection of gases comprising a concentration cell and a detection cell, and a method for producing the micro-fluidic cell for optical detection of gases in order to increase the sensitivity of optical detection of gases, selectivity of components, and accuracy of quantitative determination, and also achieve a low electric power consumption and a small-sized, light-weight configuration of the entire apparatus. It is another object of the invention to provide a gas trapping cell including a cold trap channel as the concentration cell. It is a further object of the invention to provide a gas analyzer having a micro-fluidic cell for optical detection of gases.
In a first aspect of the present invention, there is provided a micro-fluidic cell for optical detection of gases comprising:
a microchannel through which gases to be analyzed flow;
a concentration cell; and
a detection cell, and wherein
the microchannel through which gases to be analyzed flow comprises a first microchannel including a gas inlet, a second microchannel including a gas outlet, and a connecting channel which connects the first microchannel and the second microchannel,
the concentration cell has the first microchannel, a substance adapted to adsorb and desorb the gases to be analyzed, and provided in part of the first microchannel, and a heating source for heating the substance for adsorbing and desorbing the gases to be analyzed, and
the detection cell has the second microchannel, an optical fiber for entry of ultraviolet light for spectrophotometric analysis into the second microchannel, and an optical fiber for exit of the ultraviolet light for spectrophotometric analysis from the second microchannel.
In a second aspect of the present invention, there is provided a gas trapping cell comprising:
a microchannel having a gas inlet and a gas outlet port and permitting flow of gases to be analyzed;
a substance provided in part of the microchannel and adapted to adsorb and desorb the gases to be analyzed;
a heating source for heating the substance for adsorbing and desorbing the gases to be analyzed; and
a microchannel for cold-trapping the gases to be analyzed which have been desorbed from the substance for adsorption and desorption provided in part of the microchannel.
In a third aspect of the present invention, there is provided a method for producing a micro-fluidic cell for optical detection of gases, comprising the steps of:
forming a trench in a continuous pattern including an adsorbent stopper step on a top side of a first substrate;
loading a substance for adsorbing and desorbing gases into part of the trench;
connecting an optical fiber for entry of ultraviolet light for spectrophotometric analysis and an optical fiber for exit of the ultraviolet light for spectrophotometric analysis with a predetermined spacing in the trench by use of an sealing material comprising glass;
bonding a second flat substrate to the top side of the first substrate by bonding to form a microchannel in the trench; and
providing a heater on a bottom side of the first substrate and at a position corresponding to a portion loaded with the substance for adsorbing and desorbing the gases.
In a fourth aspect of the present invention, there is provided a method for producing a micro-fluidic cell for optical detection of gases, comprising the steps of:
forming a trench in a continuous pattern including an adsorbent stopper step on a top side of a first substrate;
loading a substance for adsorbing and desorbing gases into part of the trench;
forming a trench in a predetermined continuous pattern on a top side of a second substrate and a through-hole in the trench;
connecting an optical fiber for entry of ultraviolet light for spectrophotometric analysis and an optical fiber for exit of the ultraviolet light for spectrophotometric analysis with a predetermined spacing in the trench of the second substrate by use of an sealing material comprising glass;
bonding a third flat substrate to the top side of the second substrate by bonding to form a microchannel in the trench of the second substrate;
bonding a bottom side of the second substrate to the top side of the first substrate by bonding to form a microchannel in the trench of the first substrate; and
providing a heater on a bottom side of the first substrate and at a position corresponding to a portion loaded with the substance for adsorbing and desorbing the gases.
In a fifth aspect of the present invention, there is provided a method for producing a micro-fluidic cell for optical detection of gases, comprising the steps of:
forming a trench in a continuous pattern including an adsorbent stopper step on a top side of a first substrate;
loading a substance for adsorbing and desorbing gases into part of the trench;
bonding a second flat substrate having a through-hole at a position corresponding to the trench of the first substrate to the top side of the first substrate by bonding to form a microchannel in the trench of the first substrate;
providing a heater on a bottom side of the first substrate and at a position corresponding to a portion loaded with the substance for adsorbing and desorbing the gases;
forming a trench in a predetermined continuous pattern on a top side of a third substrate and a through-hole in the trench;
connecting an optical fiber for entry of ultraviolet light for spectrophotometric analysis and an optical fiber for exit of the ultraviolet light for spectrophotometric analysis with a predetermined spacing in the trench of the third substrate by use of an sealing material comprising glass;
bonding a fourth flat substrate having a through-hole at a position corresponding to the trench of the third substrate to the top side of the third substrate by bonding to form a microchannel in the trench of the third substrate; and
stacking a fifth substrate comprising a Teflon seal packing interposed between the second substrate and the third substrate and having a through-hole at a position corresponding to the through-hole of the second substrate and the through-hole of the fourth substrate.
In a sixth aspect of the present invention, there is provided an apparatus for optical detection of gases, comprising:
a micro-fluidic cell for optical detection of gases, including a microchannel through which gases to be analyzed flow, a concentration cell, and a detection cell;
a heater power source;
an ultraviolet light source;
an ultraviolet spectrophotometer; and
a controller, and wherein
the microchannel through which the gases to be analyzed flow comprises a first microchannel including a gas inlet, a second microchannel including a gas outlet, and a connecting channel which connects the first microchannel and the second microchannel,
the concentration cell has the first microchannel, a substance adapted to adsorb and desorb the gases to be analyzed, and provided in part of the first microchannel, and a heating source for heating the substance for adsorbing and desorbing the gases to be analyzed, and
the detection cell has the second microchannel, an optical fiber for entry of ultraviolet light for spectrophotometric analysis into the second microchannel, and an optical fiber for exit of the ultraviolet light for spectrophotometric analysis from the second microchannel.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.